1. Field of the Invention
This invention relates to modems, and in particular to systems and methods for reducing the processing requirements of a modem.
2. Description of the Related Art
Modems are communications devices which employ digital modulation techniques to transmit binary data over analog communications channels, e.g., telephone lines. Typically, two modems communicate over a single channel, with one modem at each end of the channel. Signal processing structures implemented at each modem provide encoding, modulation, filtering, interpolation, echo cancellation, signal detection, equalization, demodulation, and decoding functionality. Modems typically conform to international standards to ensure interoperability with modems from other manufacturers. One such standard is the V.34 specification described in ITU-T Recommendation V.34, A Modem Operating at Data Signalling Rates of up to 28 800 bits/s for Use on the General Switched Telephone Network and on Leased Point-to-Point 2-Wire Telephone-Type Circuits, dated September, 1994 (previously CCITT Recommendation V.34), which is hereby incorporated herein, in its entirety, by reference.
Traditional modem implementations include one or more dedicated digital signal processors (or DSPs) on which signal processing algorithms execute during periods of modem operations. A computer system may incorporate such a modem implementation, and in addition, typically includes application and operating system software executable on a general purpose processor (e.g., any of a variety of processors including MIPS™ R3000, R4000, and RS000 processors, processors conforming to the SPARC™, PowerPC™, Alpha™, PA-RISC™, or x86 processor architectures, etc.). Software executing on the general purpose processor sends and receives data via the modem implementation using input/output (I/O) ports, direct memory access (DMA), or other I/O structures and methods suitable for a particular general purpose processor and operating system combination.
Since a typical modem implementation includes a dedicated DSP not shared with other signal processing functions of a larger computer system, the modem's DSP and the signal processing algorithms designed to run thereon are selected and designed to meet the peak computation load of the modem. DSP cycles are either used or lost. For this reason, signal processing algorithms implementing the complete steady-state functionality of modem transmit and receive paths are typically executed on a DSP at full speed for the duration of a modem connection.
For many portable device applications such as Personal Digital Assistants (PDAs), portable computers, and cellular phones, power consumption, battery life, and overall mass are important design figures of merit. In addition, very small part counts are desirable for extremely-small, low-cost consumer devices. Modem communications are desirable in many such portable device applications. However, traditional DSP implementations of the underlying signal processing capabilities create power substantial demands, require increased part counts, and because of the power consumption of a discrete DSP, typically require larger heavier batteries.
A modem implemented as software executable on a general purpose computer could reduce part counts, power demands, and overall mass of a computer system by eliminating the DSP, its power consumption, and some of the battery capacity otherwise required. However, to fully benefit from the elimination of a DSP, such a software modem needs to co-exist with existing operating systems and applications, running on the same general purpose processor as the operating systems and applications. Unfortunately, in a computer system which includes such a software modem, the load on the general purpose processor can be significant, slowing operating system and application programs, even when the software modem is not sending or receiving data. For example, FIG. 1 depicts a 14 MIPS steady-state computational load, which is exemplary of a software implementation of a traditional modem maintaining an active connection with both transmitter and receiver structures active for the duration of the connection.